Uskhal Khan (Mongolian: Усхал; Mongolian script: ; 兀思哈勒汗), also called the Last Lord of Northern Yuan (北元末主) or by his era name the Tianyuan Emperor (天元帝), born Tögüs Temür (脫古思帖木兒; 7 March 1342 – 18 November 1388), was an emperor of the Northern Yuan dynasty, reigning from 1378 to 1388. He was the last powerful khagan of the Mongols until Dayan Khan.

Tögüs Temür was the younger brother of Biligtü Khan (Emperor Zhaozong) and son of Toghon Temür (Emperor Huizong), the last Yuan emperor. Before ascending to the Northern Yuan throne, he held the noble title of Prince of Yi (益王). He succeeded to the throne with the title of Uskhal Khan after Biligtü Khan died in 1378. During the funeral of the late emperor, the Ming court sent an embassy to participate in it and released the Northern Yuan prince, Maidarbal, who had been captured at the battle of Yingchang in 1378.

Uskhal Khan Tögüs Temür mobilized troops near Yingchang and Karakorum. He continued to press the Ming dynasty from the north, cooperating with Naghachu of the Jalayir in Manchuria. The Ming launched a massive attack on the Northern Yuan in return in 1380, sacking Karakorum.

The Ming army crushed the Northern Yuan garrison under Iliyasan in northern China in June 1380. Uskhal Khan's commanders, Öljei-Buqa and Nair-Buqa, invaded Lulun city, killing the Ming officer Liu Guang in the same year. The Ming dispatched a large army against the Northern Yuan the next year.

The Hongwu Emperor of the Ming dynasty decided to exterminate Naghachu's force in Manchuria in 1387 and two sides suffered heavier losses at a battle near Changchung city. Tögüs Temür suddenly faced a predicament in 1387 when Naghachu surrendered to the Ming due to a devastating famine. The Ming now turned their attention to the Uskhal Khan Tögüs Temür who lived in Yingchang. In 1388, he was raided by the Ming on the Buir Lake. In his escape to Karakorum, he was suddenly attacked and defeated on the Tuul River by Yesüder, a descendant of Ariq Böke, who allied with the Oirats. Yesüder's general killed the king and his son. Uskhal Khan had intended to flee westwards to join his commander Markhas. This event marked the decline of the Kublaid power and the rise of the Oirats on the Mongolian Plateau.

脫古思帖木兒（；1342年3月7日 - 1388年），是北元第三位君主，第十七位蒙古大汗，也是最後一任北元皇帝，史稱北元末主，或以他的年號天元稱為天元帝。根據元明史籍記載來看，他應是元昭宗愛猷識理達臘的弟弟。明代《北虜始末志》記載，脫古思帖木兒繼位前是益王。1378年5月13日—1388年11月1日在位，在位10年。

根據史書記載的繼承次序，脫古思帖木兒應該就是蒙古語史料中的兀思哈勒可汗（）或烏薩哈爾汗（）。《蒙古源流》和《新元史》等史料記載他是必里克圖可汗（愛猷識理達臘）的弟弟，這和《元史》中記載愛猷識理達臘的弟弟均早亡的敘述有所不同。他的蒙古文稱號是烏薩哈爾汗。無漢文廟號與諡號。胡和溫都爾在《蒙古可汗世系錄》一文中稱廟號為「益宗」，未列史源。

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